Increased atmospheric water vapor seen as key ingredient
By Summit Voice
SUMMIT COUNTY — Parts of the U.S. have seen clear statistical trends more extreme precipitation events in the past few decades, according to a new paper from the American Meteorological Society based on extensive research from federal and state agencies, as well as academic sources.
Increased water vapor in the atmosphere, as outlined by many climate change models, may be one of the key factors in the the observed changes, according to the researchers, who said that weren’t able to measure statistically significant changes in severe thunderstorms.
But for extreme precipitation, “there is strong evidence for a nationally-averaged upward trend in the frequency and intensity of events,” the paper concludes. About 76 percent of all stations reported increases in extreme precipitation.
The analysis also found the number of severe regional snowstorms since 1960 “was more than twice that of the preceding 60 years.”
While the study identified a high degree of geographic variability, they found that all regions of the U.S. have experienced a greater than normal occurrence of extreme events since 1991. “The increase is statistically significant for the U.S. as a whole and the individual regions of the Midwest and Southeast,” according to the report.
The upward trend for the 1957-2010 period is especially apparent in the Midwest, Southeast and Northeast, while the study found no significant extreme precipitation trends in the western U.S.
The report acknowledges the challenges of linking changes in extreme precipitation with global warming, but nevertheless references several papers that suggest a connection to human-caused changes in atmospheric composition — for example a paper showing that, “for the same annual or seasonal precipitation totals, warmer climates generate more extreme precipitation events compared to cooler climates.”
Here’s how the paper explains the possible links:
“This is consistent with water vapor being a critical limiting factor for the most extreme precipitation events. A number of analyses have documented significant positive trends in water vapor concentration and have linked these trends to human fingerprints in both changes of surface and atmospheric moisture.”
Several papers also document an upward trend in the number of extreme precipitation events in the vicinity of fronts associated with extra-tropical cyclones, but no evidence yet to show if there’s been an increase in the number and intensity of those fronts.
Overall, there appears to be compelling evidence that increased water vapor is a primary cause of extreme precipitation events, other factors, including changes in the characteristics of weather systems, changes in La Niña-El Niño patterns and even changes in land-use and irrigation patterns, including increased irrigation in the Great Plains.